Deprecate elasticsearch connection

[tobio]

Configuring provider connection configuration on a per-resource basis isn't supported during import operations, and duplicates the core Terraform aliases provider functionality.

Marking these attributes as deprecated as the first step to removing them in a future version of the provider.

Related to #203

[webfella]

🎉

Big 👍👍 for adding the guide for multiple deployments too mate!

[orlandothoeny]

This change makes it impossible to create a new ec_deployment instance + resources like an Elasticsearch User inside a single module without ugly workarounds

[orlandothoeny]

Got it to work using

provider "elasticstack" {
  elasticsearch {
    endpoints = [ec_deployment.deployment.elasticsearch.https_endpoint]
    username  = google_secret_manager_secret_version.root_user.secret_data
    password  = google_secret_manager_secret_version.root_password.secret_data
  }
}

However, had to manually remove the existing user resource from the state, always got an authentication error before.